1. Technical Field
This disclosure relates to a fixing device that fixes toner on a sheet by using induction heating. Also, this disclosure relates to an image forming apparatus including the fixing device that performs induction heating.
2. Description of the Related Art
Some image forming apparatuses, such as a multi-functional peripheral, a copier, a printer, and a facsimile device, perform image formation by using toner. Such an image forming apparatus includes a fixing device that fixes a toner image, which is transferred on a sheet, to the sheet by applying heat and pressure while the sheet is transported in the image forming apparatus. Some fixing devices cause a roller or a belt made of metal to generate heat by induction heating, and then apply the heat to a toner image.
An example of such a fixing device includes: alternating magnetic field generating means for generating an alternating magnetic field; a heating element that generates heat with the alternating magnetic field generated by the alternating magnetic field generating means; detecting means for detecting a value of current or voltage induced in the heating element through which the alternating magnetic field generated by the alternating magnetic field generating means passes; judging means for judging whether or not the heating element is damaged in accordance with the value of current or voltage detected by the detecting means; and protecting means for causing the alternating magnetic field generating means to stop the generation of the alternating magnetic field and protect the fixing device from being further damaged. With this configuration, damage of the heating element is detected to protect the fixing device.
The fixing device of the induction heating system includes a coil. A magnetic flux generated by the coil passes through metal or the like, and hence heat (Joule heat by eddy current) that is applied to a toner image is obtained.
An electric wire (for example, a stranded wire) wound around the coil is covered with an insulating material. However, coatings of adjacent electric wires may be broken and a short circuit may occur between the adjacent electric wires. Such a short circuit is occasionally called “layer short”. For example, a layer short may occur if expansion by a temperature increase and contraction by a temperature decrease are repeated and hence the coatings are rubbed with each other. A layer short may also occur if a scratch is made in the coating when the electric wire is wound during manufacturing of the coil or when a mechanical stress is applied to the electric wire and hence the coating is broken.
For induction heating, a resonant circuit is formed by a coil and a capacitor. The frequency of voltage applied to the resonant circuit or the frequency of current flowing through the resonant circuit is adjusted to move towards or away from a resonance frequency. Hence, the output (power) is controlled. If a layer short occurs, the potentials of two adjacent short-circuited electric wires become the same. This represents a substantial decrease in the number of turns of the electric wire wound around the coil. Hence, if a layer short occurs, characteristics of the coil, such as an inductance value, are changed. If the inductance is changed due to a layer short, the resonance frequency of the resonant circuit is changed, and the output by the induction heating can be no longer properly controlled with the control setting for the frequency before the inductance is changed. Therefore, if a layer short occurs, the temperature of a member that is heated by the coil can be no longer properly controlled.
If the fixing device is continuously used even when the inductance is changed due to a layer short, parts and circuits included in the fixing device may be broken. For example, by recognizing power applied to the resonant circuit, the switching frequency is changed so that the difference between the power and target power is eliminated (feedback control). However, if a layer short occurs, even if the feedback control is performed, the difference is not eliminated. The fixing device may be operated to endlessly change the switching frequency. Switching with an abnormal frequency may break the switching element.
The above-described fixing device does not detect a layer short, but rather detects a breakdown of a heated member (a metal belt) that is heated by the coil. As a result, the above-described problems relating to a layer short cannot be addressed. Further, the above-described fixing device needs a sensor such as an antenna, an alternating detecting circuit, a direct detecting circuit, a control circuit, etc., for detecting a breakdown of the heated member (the metal belt). Therefore, the above-described fixing device results in increased manufacturing costs.